Improve performance of unclipped save layers.

Instead of allocating a separate renderTarget and switching
between them on each draw the new implementation follows the same
pattern that the old HWUI renderer used. The area of the layer is
now copied to a buffer on the GPU, the area is then cleared, rendered
as normal, and finally the texture is redrawn using dst_over blending.

This results in no render target switches and is considerably faster
on some hardware.

Test: CtsGraphicsTestCases, CtsUiRenderingTestCases
Bug: 119222339
Change-Id: I716aac1fc31e4c7a171373d37dee82034c01cf18